During operation of a semiconductor laser device--i.e., when it is generating optical radiation-it also generates relatively large amounts of heat that must be quickly and efficiently conducted away from the device, lest its temperature rise to an undesirably high level and its useful device lifetime consequently be undesirably reduced.
U.S. Pat. No. 4,772,935 teaches a process for thermal-compression bonding of a silicon integrated circuit die to a package comprising a header. The process utilizes successive formations on a major surface of a silicon wafer (substrate), prior to its being cut into a plurality of dies, the following successive layers: (1) an adhesion layer of titanium; (2) a barrier layer, preferably of tungsten; (3) a bonding layer, preferably of gold. Also, a stress-relieving layer, preferably of gold, can be formed earlier between the adhesion layer of titanium and the major surface of the wafer. Thereafter, the substrate is cut into the plurality of dies, each of which is bonded, for example, to a ceramic header ("submount"). Prior to the die's thus being bonded to the header, a major surface of the header is coated with a layer of gold that, in turn, is coated with a "binding" layer of solder, preferably a gold-tin eutectic in order to supply a desirable bonding material for the ultimate bonds (joint) between the substrate and the header.
The purpose of the aforementioned adhesion layer is to promote adhesion of the tungsten barrier layer to the substrate. The stated purpose of the barrier layer is to suppress migration of silicon from the substrate into the originally eutectic binding layer of solder: such migration would cause an undesirable increase in the melting temperature of the layer of solder and hence a required undesirably high temperature rise in the wafer during the thermal-compression bonding process (during which the binding layer of solder must be melted to wet the surface to be bonded). The stated purpose of the bonding layer of gold is to protect the barrier layer from oxidation that would weaken the resulting bond.
In the aforementioned patent, the thickness of the layer of solder was reported to be 0.5 mil to 1.0 mil--i.e., approximately 13 .mu.m to 25 .mu.m. Such a relatively large thickness, regardless of relatively small thicknesses of the other layers, is not desirable in the context of bonding a relatively high power (over 100 milliwatt) laser to a submount: such a laser requires a significantly higher thermal conductance, and hence significantly smaller thickness, of the entire resulting bond between the laser and its submount.
However, when a gold-tin solder layer is made desirably thin from the standpoint of good and sufficient thermal conductance--i.e., about 4 or 5 .mu.m or less--then exposed surface regions of the solder layer suffer from premature freezing (solidification) during the bonding process when the solder is heated above its melting (=freezing) temperature, namely above 280.degree. C. in cases where the gold-tin solder has a eutectic composition (gold: 80 per centum by weight; tin: 20 per centum by weight) and hence has a desirable minimum melting temperature. This premature freezing is caused by migration of tin away from the exposed surface of the solder layer (initially having a eutectic or even a tin-rich nearly eutectic composition), whereby (because the solder's surface regions no longer have the eutectic or nearly eutectic compositions) the melting point of the solder's surface regions dramatically increases. More specifically, as the tin component decreases in the neighborhood of the eutectic composition on its tin-poor side, the melting temperature initially increases by about 30.degree. C. per centum (by weight) decrease in the tin component. Consequently, surface regions of the solder undesirably solidify ("freeze") during bonding, because the bonding process cannot be performed at a temperature that is high enough to prevent this freezing and at the same time that is low enough so as not to injure the device during this bonding procedure. The thinner the solder layer, the more readily it prematurely freezes: the relative composition of a thinner layer is more sensitive to absolute changes of a component than is a thicker layer.
The aforementioned premature freezing of the solder is undesirable because it causes poor "wetting" of the surface of the gold bonding layer by the solder and consequently poor bonding of the device to the submount. Thus, during subsequent device operation, the resulting poor thermal conductance of the resulting bond (caused by the poor "wetting") tends to cause injurious overheating of the device, and the resulting poor mechanical adhesion property of the bond tends to allow the device to detach from the submount.
On the other hand, other solders--such as indium, lead-tin, lead-indium, tin-indium, lead-indium-silver--have lower melting temperatures than that of a gold-tin eutectic, and hence their use would not injure the device during bonding. But, they have undesirably much lower Young's moduli of elasticity as compared with that of gold-tin solder (by a factor of 10 or more), and hence these other solders tend to produce bonds that are mechanically less stable (rigid). Also bonds made from these other solders tend to have undesirably higher creep, such higher creep being associated with the relatively low melting temperatures of these other solders as compared with that (280.degree. C. or more) of a gold-tin solder.
Another shortcoming with the technique described in the aforementioned patent stems from poor adhesion of the tungsten barrier layer to the gold-tin solder. Therefore it would be desirable to have a bonding method, preferably using gold-tin solder, that mitigates the above-mentioned shortcomings of prior art.